There is a need to detect, locate and identify underground or buried objects such as mines, buried waste and utility structures. Existing techniques for detecting, locating and identifying underground objects include techniques employing ground-penetrating radar and solar thermal imaging. While ground-penetrating radar has proven to be useful in detecting underground objects, it suffers from limited resolution, which makes the locating and identification of underground objects difficult. Solar thermal imaging, which utilizes thermal images from solar heating of the ground, is limited in that sunlight does not penetrate the ground to any appreciable depth, and it can only be used during certain hours when sunlight levels are changing. Further, the captured image is somewhat weak, because it is produced entirely by heat transfer, and is noisy due to uneven absorption of sunlight at the surface.
Recently, the use of microwave-enhanced thermal imaging, or thermography, has been proposed for use in locating buried objects such as land mines. Generally, microwave-enhanced thermography involves directing a high-energy microwave signal onto the surface of an area of interest from an aerially suspended microwave antenna, and utilizing an infrared camera to capture an image of the resulting heating. This image is compared with an infrared image of the same area prior to such microwave heating, and the difference between the images is analyzed for indications of buried objects. This technique relies on the reflection of the microwave signal from the object back toward the surface and the resulting pattern of interference with the incident microwave signal. When the heating cycle is of the proper duration, the interference pattern can be detected by the infrared camera as a corresponding temperature distribution on the surface. The benefits of microwave-enhanced thermography include relatively high resolution, control over the heating cycle, and speed of analysis.
It has been noted that the microwave-enhanced thermography technique as described above can work well when the surface is relatively smooth, but its performance suffers if the surface has appreciable irregularity or roughness. It is believed that this degraded performance arises from distortions in the microwave field caused by the irregularities, resulting in variations in the heating of the ground that are greater than those associated with the buried object(s). This effect has been referred to as xe2x80x9cnoisexe2x80x9d or xe2x80x9cclutterxe2x80x9d induced by surface roughness. If this noise is sufficiently high, it becomes much more difficult to reliably detect signal features caused by a buried object, resulting in missed detections and false alarms.
It would be desirable to enable microwave-enhanced thermography to be used in areas having appreciable surface roughness without the degraded performance that has heretofore been observed.
In accordance with the present invention, technique for performing microwave-enhanced infrared thermography is disclosed that performs well even in the presence of surface roughness.
The disclosed technique is based on the observation that the heating of the surface area is primarily dependent upon the field strength at the surface, which is substantially independent of the frequency of the microwave energy. However, the pattern of mixing of the reflected field from a buried object with the incident field varies with frequency, due to differences in the relative phase shifts caused by the propagation of the signals from the surface down to the object and back to the surface. These different mixing patterns induce corresponding different temperature patterns on the surface, which are subtracted. The result is a difference image with reduced noise from surface roughness due to the canceling effect of the subtraction.
Thus, in the disclosed thermography method, a first infrared image of a target surface area of interest is captured prior to any microwave heating. Then the target area is heated using microwaves of a first frequency, and a first xe2x80x9cheatedxe2x80x9d infrared image is captured. These two images are subtracted to form a first xe2x80x9ctemperature risexe2x80x9d image showing a pattern of temperature rise on the surface as a result of this first heating cycle.
Subsequently, the target area is heated using a different microwave frequency, and a second heated infrared image is captured. The second heated infrared image is used in calculating a second temperature rise image. The first temperature rise image from the first heating cycle is subtracted from the second temperature rise image of the second heating cycle, and the difference image is subject to image analysis to identify any characteristics indicating the presence of a buried object in the target area. The microwave signals have respective frequencies that are suitably widely spaced to maximize the difference in phase shifts and therefore the interference patterns of the two heating cycles, to yield an acceptably high signal-to-noise ratio in the difference image.
Other aspects, features, and advantages of the present invention will be apparent from the detailed description that follows.